In recent years, a solar cell (CIS solar cell) comprising a chalcopyrite semiconductor layer such as a layer of CuInSe2 and a solid solution of Ga in CuInSe2, i.e. Cu(In,Ga)Se2 attracts considerable attention as a solar cell having high photoelectric conversion efficiency. A CIS solar cell generally comprises a substrate, a back electrode layer, a chalcopyrite semiconductor layer as a light absorbing layer, a buffer layer, a transparent electrode layer, and an extraction electrode in that order.
Conventionally, glass (soda-lime glass) is used as a substrate for a CIS solar cell, and molybdenum (Mo) is used as a back electrode formed thereon. In addition, a CIS solar cell comprising a flexible film as a substrate has been proposed (See Patent Document 1, for example). As compared with a conventional solar cell comprising a glass substrate, a solar cell comprising a flexible substrate is expected to find widespread application in view of its flexibility and light-weight. Another advantage of a flexible substrate is that a solar cell may be produced in a roll-to-roll process, which is suitable for mass production.
However, a CIS solar cell comprising a flexible substrate such as a polyimide substrate is apt to have lower conversion efficiency than a CIS solar cell comprising a glass substrate. This is because even a polyimide, which is known as a heat-resistant resin, may withstand a temperature of about 450° C. at the highest and may not be heated at a temperature equal to or higher than 450° C., while a heat treatment at a temperature equal to or higher than 450° C., preferably at a temperature equal to or higher than 500° C., is required for the formation of a high-quality CIS semiconductor thin film having low defect density.
Patent Document 2 discloses a process for producing a CIS solar cell comprising a polyimide substrate and having high conversion efficiency, which comprises steps of:
forming an electrode film on a polyimide substrate;
forming a thin film containing Cu, and In and/or Ga, and Se and/or S on or over the electrode film (in other words, directly or indirectly on the electrode film); and
rapidly heating the thin film to a temperature equal to or higher than 450° C., preferably from 500° C. to 600° C., and maintaining the thin film at the temperature for 10 sec to 300 sec, thereby forming a chalcopyrite semiconductor film.
Patent Document 2 also discloses a process for producing a CIS solar cell, which comprises steps of:
forming an electrode film on a polyimide substrate;
forming a thin film containing Cu, and In and/or Ga on or over the electrode film; and
rapidly heating the thin film to a temperature equal to or higher than 450° C., preferably from 500° C. to 600° C., and maintaining the thin film at the temperature for 10 sec to 300 sec in an atmosphere containing Se and/or S, thereby forming a chalcopyrite semiconductor film.
In these processes, the step of forming a thin film which is a precursor of a semiconductor film and the step of heating the precursor thin film are separately performed, rapid heating is performed in the step of heating the precursor thin film for crystal growth, and therefore a heat treatment at a temperature equal to or higher than 500° C. is performed in a shorter period of time so that a chalcopyrite semiconductor thin film suitable for a light absorbing layer of a solar cell may be prepared. In these processes, however, a heat treatment at a high temperature is still required, even though the heat treatment is performed in a short period of time; therefore the curling of the polyimide substrate and cracks in the electrode film and/or the semiconductor film may occur when using a conventional polyimide film.
Patent Document 3 discloses a solar cell comprising a substrate film, and a laminate comprising at least an electrode layer and a chalcopyrite semiconductor thin film which is formed on the substrate film, wherein the substrate film is a polyimide film prepared by the polycondensation of an aromatic diamine and an aromatic tetracarboxylic dianhydride, which has a thickness within a range of from 3 μm to 200 μm, an average coefficient of thermal expansion up to 300° C. within a range of from 1 ppm/° C. to 10 ppm/° C., and a tensile strength at break in the length direction of 300 MPa or higher. Patent Document 3 also discloses that a preferable substrate film is a polyimide-benzoxazole film prepared by the polycondensation of an aromatic diamine having a benzoxazole structure and an aromatic tetracarboxylic dianhydride. In Example 8, a CIS solar cell comprising a polyimide film as a substrate, which is prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine by thermal imidization, is disclosed. However, although the dimensional change up to 300° C. in a temperature-increasing step is taken into consideration, the dimensional change at elevated temperatures (up to 500° C. or higher) and in a temperature-decreasing step is not taken into consideration. The polyimide film disclosed in Patent Document 3 may not have adequate properties as a substrate for a CIS solar cell.